1. Field of the Invention
This invention relates to a liquid crystal display apparatus, and a driving circuit and driving method thereof, particularly, relates to an active matrix liquid crystal display apparatus, and a driving circuit and driving method thereof.
2. Description of the Related Art
Recently, a liquid crystal on silicon (LCOS) type liquid crystal display apparatus has been commonly used in a projector and a projection television (TV) as a major component for projecting an image on a screen.
The LCOS type liquid crystal display apparatus is formed in a structure of layering with a transparent electrode, a liquid crystal layer, a reflection electrode disposed in matrix, and a liquid crystal driving element formed with a liquid crystal driving circuit on a silicon circuit board.
FIG. 22(a) is one example of a fundamental constitutional diagram of a liquid crystal driving element used in a conventional liquid crystal display apparatus according to the prior art.
FIG. 22(b) is a partially enlarged block diagram of the liquid crystal driving element showing an elliptical area “Z” in FIG. 22(a).
FIG. 23 is one exemplary block diagram of a liquid crystal element constituting a pixel of a conventional liquid crystal display apparatus according to the prior art.
The liquid crystal driving element shown in FIG. 22(a) is composed of a horizontal driver circuit 310, a vertical driver circuit 320, a horizontal signal line 305 that supplies an image signal 71 inputted externally to each of video switches S301-1, S301-2 and S301-3 (hereinafter generically referred to as video switch S301), a pixel section 330, data lines 306-1, 306-2 and 306-3 (hereinafter generically referred to as data line 306), a common electrode line 307, and gate lines 308-1, 308-2 and 308-3 (hereinafter generically referred to as gate line 308), wherein a reference sign 375 denotes a pixel selection driving section. In FIG. 22(a), a suffix number succeeding a hyphenated reference sign such as 301-1 and 301-2 exhibits the same component but they are arranged in different sections.
Further, FIG. 22(a) shows a part of the liquid crystal driving element.
The pixel section 330 is further composed of a plurality of pixels 11-13, 21-23 and 31-33, which is disposed at each intersection of each data line and each gate line respectively. As shown in FIG. 22(b), each pixel is composed of a pixel selection transistor 302, a signal holding capacitor 303 and a reflection electrode 304 respectively. In the case of the liquid crystal driving element shown in FIG. 23, each pixel is composed of a pixel selection transistor “Q”, a signal holding capacitor Cs and a reflection electrode PE respectively. A gate and a drain terminals of the pixel selection transistor 302 or “Q” is connected to the gate line 308 or “G” that functions as a line scanning line and the data line 306 or “D” respectively.
Further, as shown in FIG. 23, a liquid crystal element is composed of the reflection electrode or pixel driving electrode PE (hereinafter generically referred to as pixel driving electrode PE), an opposed electrode or common electrode CE (hereinafter generically referred to as common electrode CE) that confronts with the pixel driving electrode PE and a liquid crystal displaying substance or liquid crystal layer LCM (hereinafter generically referred to as liquid crystal layer LCM) that is sandwiched between the pixel driving electrode PE and the common electrode CE.
In FIG. 22(a), a controller 360 provides various kinds of clock signals, which are generated so as to synchronize with the image signal 71, to the horizontal driver circuit 310 and the vertical driver circuit 320 respectively. However, a providing route of the clock signals is not shown in FIG. 22(a).
Further, by driving the data line 306 and the gate line 308 in synchronism with the image signal 71, the controller 360 conducts pixel selection involving each scanning in horizontal and vertical directions.
When one pixel disposed at an intersection of the data line 306 and the gate line 308 is selected as mentioned above, the image signal 71 inputted externally is written into the signal holding capacitor 303 by way of the video switch S301, the data line 306 and the pixel selection transistor 302 in the vertical direction disposed in each pixel. Then, the liquid crystal layer LCM is driven by the pixel driving electrode 304 that is connected to the signal holding capacitor 303.
By applying a fixed voltage Vcom to the common electrode CE and supplying various voltages in response to an image signal to the pixel driving electrode PE, the liquid crystal element shown in FIG. 23 controls percentage modulation of light of the liquid crystal layer LCM and displays as an image. Generally, an AC (alternate current) driving method results in improving reliability of a liquid crystal element in longer stability. Consequently, an AC driving method is conducted to the liquid crystal element shown in FIG. 23 by applying positive and negative voltages, which make percentage modulation of light equal in response to an image signal, alternately to the pixel driving electrode PE.
In some cases, a voltage of a common electrode is changed in synchronism with timing of driving a pixel driving electrode by positive and negative voltages for the purpose of reducing a dynamic range of an image signal. However, basic concept is the same.
In the case of the liquid crystal driving element such as one example shown in FIG. 22(a), writing an image signal into each pixel is generally conducted once a frame. In other words, by writing positive and negative image signals into the signal holding capacitor 303 or Cs alternately per one frame, the liquid crystal is driven by AC.
In addition, there exists a double speed driving method, wherein liquid crystal is driven by a frequency double the writing frequency mentioned above. In this case, the driving frequency is such that two times the writing frequency 60 Hz equals 120 Hz. In any cases, the driving frequency is not so high.
Writing an image signal into the signal holding capacitor 303 or Cs is conducted by charging or discharging the signal holding capacitor 303 or Cs in relation to parasitic capacitance between ON resistance of the video switch S301 and the data line 306 or parasitic capacitance between ON resistance of the pixel selecting transistor 302 or “Q” and the signal holding capacitor 303 or Cs. Consequently, increasing the writing frequency more is not easy in consideration of element cost.
On the other hand, in the case of a liquid crystal element, if a DC (direct current) component passing across the pixel driving electrode 304 or PE and the common electrode CE enabled to reduce to zero by driving the liquid crystal by a higher frequency, reliability of the liquid crystal display apparatus is improved in preventing from burn-in, and resulted in improving quality of displaying an image.
Various methods of preventing a written-in signal component from deteriorating have been disclosed until now. The Japanese publication of unexamined patent application No. 2006-10897 disclosed the countermeasure for reducing influence on feed-through caused by parasitical capacitance of a pixel selection transistor.
Further, the Japanese publication of unexamined patent application No. 2002-250938 disclosed the countermeasure for reducing leak current of a signal holding capacitor. However, a method of driving liquid crystal by higher frequency has not been studied.
In addition, the Japanese publication of unexamined patent application No. 2004-354742 disclosed the liquid crystal display that prevented image quality from deteriorating. According to the publication, the liquid crystal display apparatus is prevented from the generation of deterioration of image quality caused by potential variation of a common electrode line and a common electrode by alternately connecting storage capacitance of respective pixels provided at the same scanning line to a storage capacitance line corresponding to the scanning line and another storage capacitance line adjacent to the scanning line every fixed plural pieces of storage capacitance and reversing polarities of compensation voltage at every storage capacitance line.
As mentioned above, it is preferable that a liquid crystal element is driven by a higher frequency in order to improve reliability such as preventing a liquid crystal display from burn-in. However, it is rather difficult to write positive and negative image signals against a common electrode voltage alternately in higher speed due to restriction of writing time with respect to a pixel.
Accordingly, a frequency of the AC driving method has been fixed to a frame rate or two times the frame rate.
Further, in the case of the liquid crystal display disclosed in the Japanese publication of unexamined patent application No. 2004-354742, there exists a problem such that polarity of the compensating voltage can be reversed at each frame.
Furthermore, there exist another problem such that an image signal voltage requires two types of voltages, positive and negative voltages with respect to the voltage Vcom of the common electrode.